refactor(pathfinder): unify dynamic-lib subprocess probing (production, test code)

[rwgk]

Closes #1771

Follow-on to PR #1777

• Consolidate canary and test subprocess entrypoints behind a single module with a shared JSON payload protocol.
• Update load/test helpers to use the unified subprocess command and payload parsing.
• Remove the old canary/test-only subprocess modules and adjust lint config.

Special attention was paid to:

• Canary remains system-search only (no change to discovery semantics).
• JSON payload parsing stays strict and errors remain explicit.
• Subprocess cwd stays neutral to avoid source-tree import assumptions.

---

Suggested review path that minimizes context switching:

Production flow first

• cuda_pathfinder/cuda/pathfinder/_dynamic_libs/subprocess_protocol.py
  Focus: payload schema (status, abs_path), parsing errors, command builder.
• cuda_pathfinder/cuda/pathfinder/_dynamic_libs/dynamic_lib_subprocess.py
  Focus: canary vs load mode behavior, JSON output, abs_path validation.
• cuda_pathfinder/cuda/pathfinder/_dynamic_libs/load_nvidia_dynamic_lib.py
  Focus: canary subprocess invocation, timeout/error handling, JSON parsing.

Test and helper flow

• cuda_pathfinder/tests/child_load_nvidia_dynamic_lib_helper.py
  Focus: command invocation, neutral cwd, JSON parsing.
• cuda_pathfinder/tests/test_load_nvidia_dynamic_lib_subprocess.py
  Focus: subprocess command args and payload expectations.
• cuda_pathfinder/tests/test_ctk_root_discovery.py
  Focus: updated canary probe payload/command expectations.
• cuda_pathfinder/tests/test_load_nvidia_dynamic_lib.py
  Focus: not-found handling and parsed abs_path.
• cuda_pathfinder/tests/test_driver_lib_loading.py
  Focus: not-found handling and parsed abs_path.
• ruff.toml
  Focus: lint allowlist path updated.

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